As shown in FIG. 1 to 4, a conventional plug (1) is provided with a pair of inward projections (11) for engaging with the end of a fuse cap (2). At the opening place of the cap (2), the fuse can be provided or replaced. In this state, even a little downward force presses the opening cap (2), as direction of arrow in FIG. 1, the projections (11) will be easily broken (as in FIG. 4). Hence, the cap (2) is going to be released from the plug (1) and can't be used in secure.